1. Field of the Invention
The present invention relates to optical sensors, especially an optical current sensor and an optical voltage sensor which can measure a wide range of current and voltage from a DC to a high frequency AC.
2. Description of the Related Art
In recent years, developments and practical applications of an optical fiber AC sensor by using the Faraday effect have been proposed (for example, see non-patent document 1: Kurosawa “Development and Application of Optical Fiber Current Sensor”, Journal of the Institute of Electrostatics Japan, vol. 28 No. 5 (p. 251-257), 2004). Along with the development of the optical fiber AC sensor, it has been expected to realize an optical fiber DC sensor that can be applied to both a power electronics machine and a DC transmission facility/DC substation equipment using the power electronics machine.
With respect to such a DC sensor, it is necessary to detect not only a DC (component having zero frequency), but also both a DC generated from superimposed components of high frequency and a current having a short rise time (1 msec or less, in some cases, 1 μsec). However, with respect to these necessities, there are problems such as an application of technologies developed for AC, a method of setting a zero point (a method of setting an output 0 when a measured current is 0), a method of adjusting the sensitivity (adjusting a reset value of the sensitivity of output) and moreover the stabilization of set values. Regarding an intensity modulation type AC sensor, as described in non-patent document 1, these problems have been solved by using a method of calculating a degree of modulation of a received signal.
However, it is not possible to apply the method of calculating the degree of modulation to a DC. Therefore, for example, a method of using a Sagnac interferometer which is used in a fiber optic gyro (for example, see non-patent document 2: M. Takahashi, et al. “Sagnac Interferometer-type fiber-optic current sensor using single-mode fiber down leads” Technical Digest of 16th International conference on optical fiber sensor) and a method of using an optical heterodyne (for example, see non-patent document 3: Kurosawa “Study of Fundamental Characteristics of Photocurrent Transformer using Optical Heterodyne Method”, volume B117, No. 3 (p. 354-363), 1997, The Institute of Electrical Engineers of Japan, Transactions on Power and Energy) are currently developed. FIG. 8 is a drawing which shows a method described in non-patent document 2, and an outline of the method is explained below.
Light emitted from a light source is converted to be linear polarized light after passing through a coupler 1, a depolarizer and a light polarizer. The linear polarized light is divided into a pair of beams by a coupler 2, and the beams pass into a loop fiber interferometer as incident light. The beams rotate in opposite directions to each other inside a loop. The pair of the beams is propagated through a sensor fiber after being converted to circularly polarized waves by a quarter-wave plate. In this step, a magnetic field induce by a measured current is applied to the sensor fiber, and a difference of propagation velocities between the pair of the beams is caused because of the Faraday effect. The pair of the beams reaches light receiving elements after being optically multiplexed by the coupler 2, and a phase difference, in other words, an intensity difference of received light according to a current is caused. Values of the current are calculated based on this intensity difference of received light.
A system described above has a constitution in which, in order to maintain the sensitivity and in order to maintain the stability of output even if the intensity of received light fluctuates, a carrier signal is generated by modulating light by using a piezoelectric vibrator (PZT), and the carrier is further modulated because of the Faraday effect. As such, it is possible to obtain a system output by demodulating the carrier. In other words, in FIG. 8, a reference signal generator, a vibrator activating portion and a piezoelectric vibrator are provided in order to generate the modulated signal.
Moreover, in order to improve the efficiency of modulation, an appended fiber (for example, approximately 100 m) is provided. Furthermore, in order to maintain the depth of modulation so as to be a certain level, after extracting second-harmonic waves and fourth-harmonic waves included in the received signal, a ratio between second-harmonic waves and fourth-harmonic waves is calculated and a signal which is proportional to the ratio is output to a modulation circuit in order to conduct a feedback operation. Regarding a signal operation portion, it is necessary to provide a synchronous detector and the like, which are rather complex components.
In a case of using a Sagnac interferometer, it is necessary to conduct modulation because if the modulation is not conducted, the sensitivity of the system is theoretically zero when a current is small. Moreover, in this case, in order to maintain a measurement accuracy of the system, other than modulation and demodulation, it is necessary to select optical components such as a ¼ plate, a polarizer and a depolarizer which have high accuracy.
As described above, a Sagnac interferometer has the following problems.    i) A delicate and complex optical system is necessary.    ii) A complex signal operation circuit is necessary.    iii) It is difficult to improve a response speed because complex operations such as generating the carrier signal and a modulation and demodulation are conducted. In order to improve a response speed, it is necessary to increase the frequency of the carrier. However, in this case, the modulation power increases and pressure on the signal operation circuit increases too.